Room: Exhibit Hall
Purpose: A fast falloff of dose beyond the PTV extending into normal tissue is a hallmark of stereotactic radiosurgery (SRS) and radiotherapy (SRT). Dose gradient index (DGI) is a parameter which quantifies the dose falloff. Prior tables of DGIs versus target volumes have been published for body site (RTOG 0915 and 0813). The purpose of this study is to suggest guidelines for DGIs for brain SRS/SRT treatments using CyberKnife data.
Methods: 485 plans for brain cancer patients treated with CyberKnife SRS/SRT at our institution between March 2015 and March 2018 were used for this study. The treatment planning utilized MultiPlan treatment planning system (Version 5.3.0) using sequential planning and Ray-Trace calculation algorithm. All plans were generated with IRIS (lesion diameter (Dlesion)>10 mm) and fixed (Dlesion<10mm) collimators of multiple aperture sizes with full path set. The MUs per beam and node were limited to 300 and 450, respectively. All plans were normalized to PTV coverage V100%>=95%. To control the dose falloff, four shells around the target were generated with sharp dose gradient objectives. Patients were separated into 6 groups with respect of tumor size (Group I (0-1cc), II (1.1-3.0cc), III (3.1-5.0cc), IV (5.1-10.0cc), V (10.1-15.0cc), and VI (15.1-40.0cc)), and the ideal and minimal acceptable DGI was quantified for each group.
Results: Normal continuous probability distribution of DGIs for each group was assumed for statistical analysis. The mean for each group was chosen to be an ideal DGI. Minimally acceptable DGI criteria was specified to reject the lowest 10% of cases for each group. The minimal accepted DGIs were 83 (Group I), 73 (II), 65 (III), 61 (IV), 51 (V) and 34 (IV).
Conclusion: This study provides a table of DGIs for brain SRS/SRT treatments, which can be used as a tool for evaluating brain SRS/SRT plan quality.